5 8 BEHAVIOR OF THE LOWER ORGANISMS 



is no reaction, — even though these conditions do not represent the 

 optimum. But as its movements carry the animal from one region to 

 another, the environmental conditions affecting it are of course changed, 

 and some of these changes in condition act as stimuli, causing the ani- 

 mal to change its movements. If the environmental change leads 

 toward the optimum, there is no reaction, but the existing behavior is 

 continued. To a change leading away from the optimum (in either a 

 plus or minus direction), Paramecium responds by the "avoiding reac- 

 tion." This consists essentially in a return to a previous position, 

 through a backward movement, then in "trying" different directions of 

 movement till one is found which leads toward the optimum. 1 Ex- 

 pressed in a purely objective way, the animal performs movements 

 which subject it successively to many different environmental condi- 

 tions. As soon as one of the conditions thus reached is of such a char- 

 acter as to remove the cause of stimulation, the avoiding reaction ceases 

 and the infusorian continues in the condition now existing. This 

 method of reacting causes the animals to collect in certain regions (as 

 near the optimum as possible), and to avoid other regions. Thus are 

 produced the so-called positive and negative reactions. The behavior 

 may be characterized briefly as a selection from the environmental con- 

 ditions resulting from varied movements. ) 



Some details of the behavior under the different classes of stimuli 

 will be given in the next chapter. 



LITERATURE m 



On the character of the movements and reactions of Paramecium : Jennings 

 1904 h, 1899, 1901. 



